Exhaust systems are widely known and used with combustion engines. Typically, an exhaust system includes exhaust tubes that convey hot exhaust gases from the engine to other exhaust system components, such as mufflers, resonators, etc. Mufflers and resonators include acoustic chambers that cancel out sound waves carried by the exhaust gases. Although effective, these components are often relatively large in size and provide limited nose attenuation.
Attempts have been made to improve low frequency noise attenuation by either increasing muffler volume or increasing backpressure. Increasing muffler volume is disadvantageous from a cost, material, and packaging space perspective. Increasing backpressure can adversely affect engine power.
Another solution for reducing low frequency noise is to use a passive valve assembly. One disadvantage with a traditional passive throttling valve configuration is a phenomena referred to as “flutter.” Valve flutter is associated with pressure fluctuations (pressure pulses) as the passive valve begins to open, i.e. moves from a fully closed position toward an open position.
The passive valve includes a flapper valve body or vane that is positioned within the exhaust pipe, with the vane being pivotable between open and closed positions. The closed position comprises a start position for the valve where the valve body is orientated to be perpendicular to an exhaust gas flow direction. The passive valve is spring biased toward the closed position and includes a valve top to define a rest/closed position for the valve. When exhaust gas pressure is sufficient to overcome this spring bias, the vane is pivoted toward the open position.
Valve flutter results when the pressure that contributes to the opening of the valve is decreased as the valve opens. The decrease in pressure can contribute to a reduction in valve opening force, leading to the spring biasing force returning the valve to the closed position. A subsequent pressure pulse (an increase in pressure subsequently followed by a decrease in pressure) results in the flapper valve body beginning to open in response to the increase in pressure immediately followed by closing movement in response to the decrease in pressure. When a series of these pressure pulses are generated, such as when the engine is operating a low speeds for example, the valve “flutters” back and forth between opening and closing. This can result in undesirable noise generation as the flapper valve body impacts the valve stop during each closing movement. Further, these multiple impact events can cause pre-mature wear on the valve body.